Power amplification circuit having transformer

ABSTRACT

In order to realize a wider bandwidth of a frequency characteristic of a power amplification circuit, outputs of differential push-pull amplifiers which are matched at respectively different frequencies are combined together by secondary inductors, and the combined signal is outputted.

CROSS REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2008-316891 filed onDec. 12, 2008 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a power amplification circuit, inparticular to a power amplification circuit having a wide band frequencycharacteristic.

A power amplification circuit performs a power amplification of a weaksignal to a necessary level, and outputs the amplified signal. Forexample in wireless communication use as exemplified by a portabledevice, such a power amplification circuit is utilized in order toamplify a weak high frequency signal to a signal with a sufficient powerwhich a wireless system requires in outputting.

One example of the power amplification circuit includes a differentialpush-pull method. A power amplification circuit according to thedifferential push-pull method combines by a combiner a differentialsignal amplified by a pair of transistors, and generates an outputsignal. Since a differential signal is utilized, an output twice as highin amplitude as an output signal of a single transistor is obtained, andin addition, even harmonics are balanced out. Therefore, thedifferential push-pull method provides a means effective in realizing anamplification circuit of high power and low distortion.

In mobile communications fields as typified by a mobile-phone,realization of low cost when practicing reduction of occupied area is animportant subject. Therefore, as a component transistor, a miniaturizedCMOS transistor (complementary insulated-gate field effect transistor)is utilized, and as a combiner in a microwave region, a transformer isused frequently. An example of configuration of a differential push-pullamplifier utilizing such miniaturizing CMOS process is described in NonPatent Literature 1 (Jongchan Kang, et al., “A single-chip linear CMOSpower amplifier for 2.4 GHz WLAN”, International Solid-State CircuitsConference 2006, Digest of Technical Papers, pp. 761-769, February2006).

In the configuration of the power amplifier illustrated in Non PatentLiterature 1, a transformer which serves as a combiner includes aprimary slab inductor and a secondary slab inductor, having a half-turnwinding, respectively. Both ends of the primary metal slab are driven byone pair of MOS transistors which receive a differential signal. In NonPatent Literature 1, the primary inductor and secondary inductor of thetransformer have respective inductors of a half-turn winding, andimprovement of conversion efficiency (a ratio of output power Pout toinput power Pin, Pout/Pin) is promoted by reducing cancellation ofmagnetic flux from an opposing side.

Patent Literature 1 (Japanese Unexamined Patent Application Publication(Translation of PCT application) No. 2005-503679) disclosesconfiguration of a power amplification circuit using a differentialpush-pull amplifier aiming at low-loss, a small area, and high power. Inthe configuration disclosed by Patent Literature 1, outputs of pluraldifferential push-pull amplifiers are combined together by atransformer, thereby realizing a several-Watt-class output with the useof miniaturized CMOS transistors. Specifically, in Patent Literature 1,the secondary inductors of the transformer are coupled in series, andthe outputs of four differential push-pull amplifiers are combinedtogether. Each of the secondary inductors performs impedance conversionand provides low output impedance to a drain of a transistor of eachpush-pull amplifier. Accordingly, drain voltage is suppressed low andhigh-output power is realized. The primary inductor and secondaryinductor of the transformer are respectively formed in a slab shape, andthe transformer is arranged to form a circular geometry. Accordingly, apower amplification circuit with low loss and small area is realized.

Patent Literature 2 (Japanese Unexamined Patent Publication No.2006-295896) discloses configuration of a power amplifier aiming atimprovement of the efficiency and operation region of the poweramplifier. In the configuration disclosed by Patent Literature 2, atransmission line transformer used as a matching circuit of the poweramplifier utilizes a primary transmission line (inductor) of a differentshape. That is, primary inductors with a different shape and a differentparasitic component are arranged on both sides of a secondary inductorof the transformer, and a differential push-pull amplifier is coupled toeach of the primary inductors. The primary inductors are switched sothat load resistance may be small when generating a high-output power,and the load resistance may be large when generating a low-output power.In Patent Literature 2, improvement of the efficiency and operationregion (dynamic range) of the entire power amplification circuit arepromoted by providing different output load to two differentialpush-pull amplifiers.

-   -   (Patent Literature 1) Japanese Unexamined Patent Publication No.        2005-503679    -   (Patent Literature 2) Japanese Unexamined Patent Publication No.        2006-295896    -   (Non Patent Literature 1) Jongchan Kang, et al., “A single-chip        linear CMOS power amplifier for 2.4 GHz WLAN”, International        Solid-State Circuits Conference 2006, Digest of Technical        Papers, pp. 761-769, February 2006.

SUMMARY OF THE INVENTION

In the mobile communications field, a power amplifier is used in atransmission system requiring high power and low distortion. However, inthe mobile communications field, various telecommunications standardsexist, and a group of various radio transmission parameters (a frequencyband, bandwidth, a modulation method, a necessary signal-to-noise ratio,etc.) are specified; accordingly it is required to set up necessaryparameters corresponding to each specification. In particular, in atelecommunications standard with a broad frequency-band modulation andin an international standard specification according to atelecommunications standard of each country, it is required to maintainhigh power over a wide band. However, a high power CMOS transistorgenerally tends to have a narrow band frequency characteristic, sincethe output impedance of the high power CMOS transistor is low and outputmatching is provided using a matching circuit. When using a CMOSdifferential push-pull amplifier disclosed by Patent Literature 1 orPatent Literature 2, or illustrated by Non Patent Literature 1, a narrowband frequency characteristic is obtained similarly. In PatentLiterature 1 and Patent Literature 2 and Non Patent Literature 1, noconsideration is taken about configuration which provides a wide bandfrequency characteristic.

Accordingly, an object of the present invention is to provide a poweramplification circuit having a wide band frequency characteristic.

Another object of the present invention is to realize a poweramplification circuit having a wide band frequency characteristic byemploying a power amplifier which exhibits a narrow band frequencycharacteristic.

A power amplification circuit according to the present inventionincludes plural differential push-pull amplifiers matched respectivelyto different frequencies, and all the outputs of the plural differentialpush-pull amplifiers are combined together by a secondary inductor of atransformer.

The outputs of the plural differential push-pull amplifiers are combinedtogether by the secondary inductor, and can maintain high power in afrequency band specified by the respectively different matchingfrequencies of the differential push-pull amplifiers. Accordingly, it ispossible to realize a power amplification circuit which exhibits a flatfrequency characteristic over a wide band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating schematically configuration of a poweramplification circuit according to Embodiment 1 of the presentinvention;

FIG. 2 is a drawing illustrating visually magnitude relation of LCcomponents of the power amplification circuit illustrated in FIG. 1;

FIG. 3 is a drawing illustrating a simulation result of a frequencycharacteristic of the power amplification circuit according toEmbodiment 1 of the present invention;

FIG. 4 is a drawing illustrating configuration of a power amplificationcircuit according to Embodiment 2 of the present invention;

FIG. 5 is a drawing illustrating configuration of a power amplificationcircuit as a modified example of Embodiment 2 of the present invention;

FIG. 6 is a drawing illustrating a simulation result of a frequencycharacteristic of the power amplification circuit according toEmbodiment 2 of the present invention;

FIG. 7 is a drawing illustrating configuration of a power amplificationcircuit according to Embodiment 3 of the present invention;

FIG. 8 is a drawing illustrating schematically an example of structureof a transformer employed in the power amplification circuit accordingto Embodiment 3 of the present invention;

FIG. 9 is a drawing illustrating schematically structure of atransformer as a modified example, employed in the power amplificationcircuit according to Embodiment 3 of the present invention;

FIG. 10 is a drawing illustrating configuration of a power amplificationcircuit according to Embodiment 4 of the present invention;

FIG. 11 is a drawing illustrating schematically structure of atransformer as a modified example, employed in the power amplificationcircuit according to Embodiment 4 of the present invention; and

FIG. 12 is a drawing illustrating schematically configuration of a poweramplification circuit according to Embodiment 5 of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 illustrates configuration of a power amplification circuitaccording to Embodiment 1 of the present invention. In FIG. 1,differential input signals IN(+) and IN(−) are supplied to inputterminals 1 and 2, respectively. Differential push-pull amplifiersPA1-PAn are coupled in parallel to the input terminals 1 and 2. Thedifferential push-pull amplifiers PA1-PAn are respectively matched atdifferent frequencies f1, f2, . . . fn. Here the frequencies f1, f2, . .. fn satisfy the relationship of f1<f2< . . . <fn.

Each of the differential push-pull amplifiers PA1-PAn includesamplifiers which are provided corresponding to the respective inputterminals 1 and 2, and a parallel resonant circuit of a capacitor and aprimary inductor, which performs matching of the outputs of theamplifiers. Specifically, the differential push-pull amplifier PA1includes amplifiers AMP11 and AMP12 which are provided to the inputterminals 1 and 2, respectively, and a capacitor C1 and a primaryinductor L11 which are coupled in parallel between outputs of theamplifiers AMP11 and AMP12. The differential push-pull amplifier PA2includes amplifiers AMP21 and AMP22 which are provided to the inputterminals 1 and 2, respectively, and a capacitor C2 and a primaryinductor L21 which are coupled in parallel between outputs of theamplifiers AMP21 and AMP22. The differential push-pull amplifier PAnincludes amplifiers AMPn1 and AMPn2 which are provided to the inputterminals 1 and 2, respectively, and a capacitor Cn and a primaryinductor Ln1 which are coupled in parallel between outputs of theamplifiers AMPn1 and AMPn2.

In a differential push-pull amplifier PAi (i=1, . . . n), a matchingfrequency of amplifiers AMPi1 and AMPi2 is determined by a parallelresonant circuit of a capacitor Ci and a primary inductor Li1.

Secondary inductors L12, L22-Ln2 are provided in opposed positions tothe primary inductors L11, L21-Ln1 of the differential push-pullamplifiers PA1, PA2-PAn, respectively. The secondary inductors L12-Ln2are coupled in series between output terminals 3 and 4. Differentialoutput signals OUT(+) and OUT(−) are outputted from the output terminals3 and 4. Here, the symbols (+), (−) of the differential output signaldefine the amplifiers AMP11, AMP12-AMPn1, AMPn2 as a non-invertingamplifier (positive phase amplifier). A single phase signal can beoutputted from one output terminal of the output terminals 3 and 4 bygrounding the other output terminal.

The primary inductors L11-Ln1 and the corresponding secondary inductorsL12-Ln2 are included respectively in a transformer which performsimpedance matching and impedance conversion. The so-called “the polarityof a coil (indicated by a black dot)” of the primary inductors L11-Ln1and the secondary inductors L12-Ln2 is the same. Therefore, thesecondary inductors L12-Ln2 are coupled in series between the outputterminals 3 and 4, and secondary signals are respectively generated bymagnetic coupling between the primary inductors L11-Ln1 and thecorresponding secondary inductors L12-Ln2 of the differential push-pullamplifiers PA1-PAn, and combined together by the secondary inductorsL12-Ln2. Accordingly, the combined signal is outputted to the outputterminals 3 and 4.

The n differential push-pull amplifiers PA1-PAn are matched at mutuallydifferent frequencies f1-fn. The matching frequencies are respectivelydetermined by capacitance of the corresponding capacitor Ci andinductance of the corresponding primary inductor Li1. Generally, in thedifferential push-pull amplifiers PAi and PAj (PAi and PAj are differentwith each other) among the differential push-pull amplifiers PA1-PAn,capacitance of the capacitor Ci and capacitance of the capacitor Cjdiffer with each other, and inductance of the primary inductor Li1 andinductance of the primary inductor Lj1 differ with each other.

When the amplifiers AMP11, AMP12-AMPn1, AMPn2 included in thedifferential push-pull amplifiers PA1-PAn have the same characteristic,inductance L and capacitance C necessary for matching have a tendency tobecome smaller as the matching frequency becomes higher.

FIG. 2 illustrates schematically shapes of the capacitors C1-Cn and theprimary inductors L11-Ln1 of a transformer included in the differentialpush-pull amplifiers PA1-PAn, so that the magnitude relation of thecapacitance of the capacitors C1-Cn and the magnitude relation of theinductance of the primary inductors L11-Ln1 of the transformer can beunderstood visually. In FIG. 2, the magnitude of capacitance isindicated by the length of an opposing electrode and the magnitude ofinductance is indicated by the length of an inductor.

In FIG. 2, the capacitors C1-Cn are formed from the same material and bythe same process, and the capacitance thereof is proportional to anopposing area of the electrode. In FIG. 2, the opposing electrode areais indicated by the length of the electrode. When an inductor which isincluded in each transformer is formed from the same material and by thesame process, the inductance of the primary inductors L11-Ln1 of thetransformer increases monotonically with the length thereof, regardlessof whether the inductor is formed by a coil or a metal slab. Therefore,as illustrated in FIG. 2, in the differential push-pull amplifier PA1matched at the lowest frequency f1, the opposing electrode area of thecapacitor C1 is the greatest, and the length of the primary inductor L11of the transformer is the longest. As the matching frequency becomeshigher, the opposing electrode area of the capacitors C2, . . . Cnbecomes smaller in order, and the length of the primary inductors L21, .. . Ln1 of the transformer becomes shorter in order.

Therefore, when the amplifiers AMP11, AMP12-AMPn1, AMPn2 have the samecharacteristic, it is possible to realize the differential push-pullamplifiers which are matched at mutually different frequencies, byadjusting the opposing electrode area of the capacitor and the length ofthe primary inductor of the transformer.

In the configuration illustrated in FIG. 2, each of the secondaryinductors L12-Ln2, arranged in opposed positions to the primaryinductors L11-Ln1 of the transformer, is set as the same length as thecorresponding primary inductor. In such a case, the turn ratio of eachprimary inductor to secondary inductor is set equivalently equal;accordingly, the impedance conversion ratio thereof is set to one. Allimpedance conversion ratios of the differential push-pull amplifiersPA1-PAn are made equal, and output signals, each matched to the outputload in the secondary inductors L12-Ln2, can be combined together andthe combined output signal can be generated at the output terminals 3and 4. Accordingly, even in a case where frequency of the differentialinput signals IN(+) and IN(−) supplied to the input terminals 1 and 2 isdifferent, it is possible to generate a large output signal by adifferential push-pull amplifier which is matched to the frequency ofthe input signal, and it is also possible to obtain the frequencycharacteristic of the output signal which has peaks at frequencies f1-fnat which these differential push-pull amplifiers PA1-PAn are matched.Therefore, the frequency characteristic of a wider bandwidth can berealized.

FIG. 3 illustrates a simulation result of a frequency characteristic ofan output signal of the power amplification circuit according toEmbodiment 1 of the present invention. FIG. 3 illustrates an outputfrequency characteristic in a case where four differential push-pullamplifiers (n=4) are provided. In FIG. 3, the horizontal axis indicatesa frequency (in units of GHz), and the vertical axis indicates an output(in units of dBm).

As illustrated in FIG. 3, since the output signals of the differentialpush-pull amplifiers PA1-PA4 matched at frequencies f1-f4 respectivelyare combined together by the secondary inductors, the output signalshaving peaks at the frequencies f1-f4, respectively, are combinedtogether; accordingly, the frequency characteristic of a wider bandwidthis realized, through the superposition of mutually different pluralpeaks.

Therefore, even in a case where the output frequency characteristic ofeach of the differential push-pull amplifiers PA1-PAn has a narrow band,by combining together all the output signals of these differentialpush-pull amplifiers PA1-PAn by the secondary inductors of thetransformer, a power amplification circuit which has a wide bandfrequency characteristic can be realized.

Embodiment 2

FIG. 4 illustrates schematically configuration of a power amplificationcircuit according to Embodiment 2 of the present invention. Theconfiguration of the power amplification circuit illustrated in FIG. 4differs from the configuration of the power amplification circuitaccording to Embodiment 1 illustrated in FIG. 1 in the following points.That is, in each of the differential push-pull amplifiers PA1-PAn,amplifiers arranged to the input terminals 1 and 2 are formedrespectively by a series body of a former-stage amplifier plus asucceeding-stage amplifier. Specifically, in the differential push-pullamplifier PA1, a series body of a former-stage amplifier FP11 plus asucceeding-stage amplifier SP11 is provided to the input terminal 1, anda series body of a former-stage amplifier FP12 plus a succeeding-stageamplifier SP12 is provided to the input terminal 2. In the differentialpush-pull amplifier PA(n−1), a series body of a former-stage amplifierFP(n−1)1 plus a succeeding-stage amplifier SP(n−1)1 is provided to theinput terminal 1, and a series body of a former-stage amplifier FP(n−1)2plus a succeeding-stage amplifier SP(n−1)2 is provided to the inputterminal 2. In the differential push-pull amplifier PAn, a series bodyof a former-stage amplifier FPn1 plus a succeeding-stage amplifier SPn1is provided to the input terminal 1, and a series body of a former-stageamplifier FPn2 plus a succeeding-stage amplifier SPn2 is provided to theinput terminal 2.

All the former-stage amplifiers FP11, FP12-FP(n−1)1, FP(n−1)2, to FPn1,FPn2 have the same operating characteristic. The succeeding-stageamplifiers SP11, SP12-SPn1, SPn2 are formed by an inverting amplifier(negative phase amplifier), and have the same operating characteristic.

In the differential push-pull amplifiers PA1-PA(n−1), feedback resistorsR11-R(n−1)1 are coupled between each output and input of thesucceeding-stage amplifiers SP11-SP(n−1)1, and feedback resistorsR12-R(n−1)2 are coupled between each output and input of thesucceeding-stage amplifier SP12-SP(n−1)2. Such a feedback resistor isnot provided in the succeeding-stage amplifiers SPn1 and SPn2 of thedifferential push-pull amplifier PAn.

In each of the differential push-pull amplifiers PA1-PA(n−1), the valuesof resistance of the feedback resistors Ra1 and Ra2 are equal, and thedegree of negative feedback of the succeeding-stage amplifiers SPa1 andSPa2 is set equal. Here, “a” is one number of 1 to (n−1).

A capacitor Ci and a primary inductor Li1 of a transformer are coupledbetween the outputs of the succeeding-stage amplifiers SPi1 and SPi2(i=1, . . . n). Arrangement of the transformer and the capacitor whichperform output matching is the same as the configuration of the poweramplifier illustrated in FIG. 1. Therefore, the same reference symbol isattached to a corresponding part, and the detailed explanation thereofis omitted. However, since the succeeding-stage amplifiers SP11,SP12-SPn1, SPn2 are inverting amplifiers (negative phase amplifiers),when the former-stage amplifiers FP11, FP12-FPn1, FPn2 are non-invertingamplifiers (positive phase amplifiers), a signal of reversed phase isoutputted to the output terminals 4 and 5, compared with the case ofEmbodiment 1. That is, the output signal OUT(−) is outputted to theoutput terminal 4, and the output signal OUT(+) is outputted to theoutput terminal 5. Also in the present case, similarly to Embodiment 1,either of the output terminals 4 and 5 may be grounded, and a singlephase signal may be generated from the other output terminal which isnot grounded.

Generally, gain of an amplifier has frequency dependence due to theoperating characteristic etc. of a transistor included therein, anddecreases monotonically as the frequency increases toward an upper limitfrequency. Therefore, an output signal at the frequency f1 of thedifferential push-pull amplifier PA1 tends to become greater than anoutput signal at the frequency f2 (>f1) of the differential push-pullamplifier PA2. Similarly, an output signal at the frequency f(n−1) ofthe differential push-pull amplifier PA(n−1) tends to become greaterthan an output signal at the frequency fn of the differential push-pullamplifier PAn.

In this case, when the output signals of the differential push-pullamplifiers PA1-PAn are simply combined together, the resultant outputfrequency characteristic shows that the output power decreases as thefrequency becomes higher, as illustrated in FIG. 3.

In order to make such a frequency characteristic flat, what is necessaryis just to suppress the outputs of the differential amplifiersPA1-PA(n−1) in accordance with the output of the differential push-pullamplifier PAn. The method to cope with the issue includes a method ofcoupling a series resistor to an input of an amplifier to attenuate aninput signal, and a method of suppressing gain of an amplifier bynegative feedback via a resistance element. In contrast to theattenuation of an input signal which only reduces an output of theamplifier, the negative feedback exhibits an effect to realize a widerbandwidth of the frequency characteristic of the amplifier in additionto gain suppression. Therefore, by realizing the wider bandwidth of thefrequency characteristic of a single differential push-pull amplifier bynegative feedback, it is possible to make flatter the frequencycharacteristic of the entire amplifier.

Specifically, feedback resistors R11, R12-R(n−1)1, R(n−1)2 are coupledto the succeeding-stage amplifiers SP11, SP12-SP(n−1)1, SP(n−1)2,respectively, and negative feedback is applied to the input via theresistance element to these succeeding-stage amplifiers SP11,SP12-SP(n−1)1, SP(n−1)2; accordingly an output power is suppressed. Byuse of the scheme, the output gain of the differential push-pullamplifiers PA1-PA(n−1) is fitted to (is set approximately equal to) theoutput gain of the differential push-pull amplifier PAn which has thehighest matching frequency, and the output frequency characteristic ismade flat.

As a general trend, the value of resistance of negative feedbackresistors Rk1 and Rk2 of a differential push-pull amplifier PAk becomessmaller than the value of resistance of the feedback resistors Rj1 andRj2 of a push-pull amplifier PAj matched at frequency fj (>fk: j=k+1).In this case, the degree of negative feedback by the feedback resistorsRj1 and Rj2 of the succeeding-stage amplifiers SPj1 and SPj2 is madesmaller than the degree of negative feedback applied to thesucceeding-stage amplifiers SPk1 and SPk2. That is, the value ofresistance of the feedback resistor is sequentially enlarged as thematching frequency becomes higher, and the degree of negative feedbackis sequentially made smaller.

Since no feedback resistor is provided in the differential push-pullamplifier PAn matched at the frequency fn, the outputs of thedifferential push-pull amplifiers PA1-PA(n−1) are made smaller byapplying negative feedback, so as to correspond to the output of thedifferential push-pull amplifier PAn of which the gain is the smallest.Since the outputs of the differential push-pull amplifiers PA1-PAn arecombined together by a series body of the secondary inductors L12-Ln2,it is possible to make flat the frequency characteristic of the poweramplification circuit which comprises these differential push-pullamplifiers PA1-PAn.

The values of resistance of the feedback resistors R11, R12-R(n−1)1,R(n−1)2 may be the same. When the power of the output signal is large,large negative feedback is applied and the degree that the output poweris suppressed becomes large. The value of resistance of these negativefeedback resistors may be suitably determined corresponding to thefrequency dependence of the output power.

(Modification)

FIG. 5 illustrates configuration of a modified example of a poweramplification circuit according to Embodiment 2 of the presentinvention. The configuration of the power amplification circuitillustrated in FIG. 5 differs from the configuration of the poweramplification circuit illustrated in FIG. 4 in the following points.

That is, in each of the differential push-pull amplifiers PA1-PA(n−1), anon-inverting amplifier (positive phase amplifier) is utilized, assubstitute for the inverting amplifier (negative phase amplifier). Thatis, succeeding-stage amplifiers SA11 and SA12 are provided in thedifferential push-pull amplifier PA1, and succeeding-stage amplifiersSA(n−1)1 and SA(n−1)2 are provided in the differential push-pullamplifier PA(n−1). Succeeding-stage amplifiers SAn1 and SAn2 areprovided also in the differential push-pull amplifier PAn.Succeeding-stage amplifiers SAj1 and SAj2 are provided also in anot-shown differential push-pull amplifier PAj (j=2, . . . (n−2)). Allof the succeeding-stage amplifiers SA11, SA12-SAn1, SAn2 have the sameoperating characteristic. The former-stage amplifiers FP11, FP12-FPn1,FPn2 have the same operating characteristic also.

In order to apply negative feedback to an output, in each of thedifferential push-pull amplifiers PA1-PA(n−1), the input and the outputof the succeeding-stage amplifier are cross-coupled by a resistor. Thatis, in the differential push-pull amplifier PAi (i=1, . . . (n−1)), aresistor Zi1 is coupled between the output of the succeeding-stageamplifier SAi1 and the input of the succeeding-stage amplifier SAi2, anda resistor Zi2 is coupled between the output of the succeeding-stageamplifier SAi2 and the input of the succeeding-stage amplifier SAi1.Relationship of the value of resistance of the feedback resistiveelements Z11, Z12-Z(n−1)1, Z(n−1)2 is the same as that of the resistorsR11, R12-R(n−1)1, R(n−1)2 in the power amplification circuit previouslyillustrated in FIG. 4.

The other configuration of the power amplification circuit illustratedin FIG. 5 is the same as that of the power amplification circuitillustrated in FIG. 4. Therefore, the same reference number is attachedto a corresponding part, and the detailed explanation thereof isomitted. However, since a non-inverting amplifier (positive phaseamplifier) is utilized as the succeeding-stage amplifier in each of thedifferential push-pull amplifiers PA1-PAn, output signals OUT(+) andOUT(−) are generated at the output terminals 4 and 5, respectively. Theobtained output signals OUT(+) and OUT(−) are in phase with the inputsignals IN(+) and IN(−) supplied to the input terminals 1 and 2.

In the configuration of the power amplification circuit illustrated inFIG. 5, each of the differential push-pull amplifiers PA1-PA(n−1)amplifies, in the respective interior, the differential signals IN(+)and IN(−) which are supplied to the input terminals 1 and 2, generatesthe differential signals and drives the corresponding primary inductor.In the differential push-pull amplifier PAi, the output signal of thesucceeding-stage amplifier SAi1 and the output signal of thesucceeding-stage amplifier SAi2 are in reversed phase. Therefore, byproviding the feedback resistors Zi1 and Zi2 in cross-coupling, it ispossible to apply negative feedback to the inputs of thesucceeding-stage amplifiers SAi1 and SAi2; accordingly, it is possibleto suppress the output signal.

Therefore, also in the configuration illustrated in FIG. 5, by applyingto each input negative feedback corresponding to the output, it ispossible to control the output power almost the same, even in a casewhere each of the differential push-pull amplifiers PA1-PAn is matchedat mutually different frequencies f1-fn. Accordingly, it is possible toobtain a flat frequency characteristic over a wide band.

FIG. 6 illustrates a simulation result of a frequency characteristic ofan output signal of the power amplification circuit according toEmbodiment 2 of the present invention. In FIG. 6, the horizontal axisindicates a frequency (in units of GHz), and the vertical axis indicatesan output (in units of dBm). As the simulation conditions, it is assumedthat four differential push-pull amplifiers are employed and that a gainof a former-stage amplifier is one. The simulation is performed on thesame conditions as the simulation illustrated in FIG. 3 except that anegative feedback resistance is added. The configuration of poweramplification circuit illustrated in FIG. 5 exhibits a simulatedfrequency characteristic similar to the frequency characteristic of thepower amplification circuit illustrated in FIG. 4 where the invertingamplifiers are utilized as the succeeding-stage amplifiers.

As illustrated in FIG. 6, compared with the output frequencycharacteristic illustrated in FIG. 3, three peaks in a lower frequencyregion (outputs corresponding to the frequencies f1-f3) are suppressed,resulting in a flatter frequency characteristic. Furthermore, a widerbandwidth of the frequency characteristic is realized by the negativefeedback, for each single body of the differential push-pull amplifiers(PA1-PA3) corresponding to the frequencies f1-f3. In addition, a furtherflatter frequency characteristic is realized in the lower frequencyregion, because the amount of the negative feedback is larger in thelower frequency.

Some amplifiers, while performing an invertion-amplification in a lowfrequency region, may make the phase difference between an input signaland an output signal smaller than π/2 in a high frequency region, due toa parasitic component of the amplifiers. In such a case, the invertingamplifiers with such poor RF response characteristics are regarded asnon-inverting amplifiers and the feedback resistors need to be coupledin cross as illustrated in FIG. 5 instead of FIG. 4.

As described above, according to Embodiment 2 of the present invention,a resistance element is coupled so that negative feedback may be appliedto the output of the internal amplifier in the differential push-pullamplifiers other than the differential push-pull amplifier which ismatched at the highest frequency. Accordingly, the output gain of eachof the differential push-pull amplifiers is equalized, and it becomespossible to obtain a power amplification circuit which has a flatfrequency characteristic over a wide band.

Embodiment 3

FIG. 7 illustrates schematically configuration of a power amplificationcircuit according to Embodiment 3 of the present invention. In the poweramplification circuit illustrated in FIG. 7, a transformer 10 whichperforms matching an combining of outputs of differential push-pullamplifiers is provided with primary inductors L11-Ln1 arranged inparallel and a secondary inductor L2 arranged in common to the primaryinductors L11-Ln1. The secondary inductor L2 is coupled between outputterminals 4 and 5.

Corresponding to the primary inductors L11-Ln1, capacitors C1-Cn,amplifiers AMP11-AMPn1, and amplifiers AMP12-AMPn2 are arrangedsimilarly as in Embodiment 1 illustrated in FIG. 1. Since the primaryinductors L11-Ln1 are arranged in parallel with each other in thepresent configuration, the amplifiers AMP11-AMPn1 coupled to the inputterminal 1 and the amplifiers AMP12-AMPn2 coupled to the input terminal2 are grouped respectively and arranged separately on one side of and onthe other side of the transformer.

Also in the present configuration of the power amplification circuitillustrated FIG. 7, each differential push-pull amplifier includesamplifiers AMPi1 and AMPi2 in pair, and is matched at mutually differentfrequencies f1-fn.

Also in the configuration illustrated in FIG. 7, the primary inductorsL11-Ln1 arranged in parallel with the same polarity are magneticallycoupled to the secondary inductor L2 in common, therefore, the outputsof the differential push-pull amplifiers (PA1-PAn) are combined togetherby the secondary inductor L2, to realize a wide band frequencycharacteristic.

In the configuration illustrated in FIG. 7, the secondary inductor L2 isprovided common to the primary inductors L11-Ln1 coupled to the pluraldifferential push-pull amplifiers (PA1-PAn). Therefore, compared withthe configuration in which plural secondary inductors are providedcorresponding to each of the primary inductors L11-Ln1, and are coupledin series, the layout area of the transformer 10 can be reduced;therefore, when the power amplification circuit is formed by one chip,the chip area can be made small.

In an analog circuit, inductance is dependent on length, width, etc. ofan inductor, therefore, it is difficult to realize high performance byprocess miniaturization. This fact applies equally to the transformer 10as an output unit which includes inductors. Therefore, a large effect isexpected for a chip area reduction by common use of the secondaryinductor of the output-combining transformer to the plural differentialpush-pull amplifiers (PA1-PAn), and by reducing the occupied area of thetransformer 10 to 1/n times substantially.

In the configuration of the power amplification circuit illustrated inFIG. 7, as in the configuration according to Embodiment 2, eachdifferential push-pull amplifier may adopt a former-stage amplifier anda succeeding-stage amplifier arranged in series with a negative feedbackresistive element, in lieu of the amplifiers AMPi1 and AMPi2. In theconfiguration, it is possible to realize a flat frequency characteristicsimilarly as in Embodiment 2.

In the power amplification circuit illustrated in FIG. 7, the primaryinductors are disposed, from the left of the figure, in order from theprimary inductor L11 having the matching frequency f1 to the primaryinductor Ln1 having the highest matching frequency fn. However, theprimary inductors may be disposed in the reversed order.

FIG. 8 illustrates schematically an example of configuration of thetransformer 10 employed in the power amplification circuit according toEmbodiment 3 of the present invention. FIG. 8 illustrates arrangement ofthe inductors of the transformer 10 which corresponds to fourdifferential push-pull amplifiers (PA1-PA4).

In FIG. 8, primary inductors 30-33 each having the shape of a loop witha part separated (separation ends) are arranged concentrically in order.Each of the loop-shaped primary inductors (inductor loop) 30-33 isformed by a metallic wiring with the same line width. A first separationend and a second separation end of each loop-shaped primary inductor arecoupled to the output of a corresponding amplifier which generates adifferential signal.

As for the loop-shaped primary inductors 30-33, the length is increasedfrom the inside toward the outside in order. Accordingly, when the linewidth is equal, the inductance is increased in order from theloop-shaped primary inductor 30 to the loop-shaped primary inductor 33.Therefore, the innermost loop-shaped primary inductor 30 corresponds tothe primary inductor L14 with the highest matching frequency, and theloop-shaped primary inductor 33 corresponds to the primary inductor L11with the lowest matching frequency. An inductor (inductor loop) 35 ofthe shape of a loop with a part separated (separation ends) is arrangedconcentrically at the outer circumference of the loop-shaped primaryinductor 33. Both separation ends of the loop-shaped secondary inductor35 (L2) are arranged in an opposed position to the separation ends ofeach of the primary inductors 30-33, and are coupled to the outputterminals 4 and 5, respectively.

In the arrangement illustrated in FIG. 8, the inductors are formed usingmetallic wiring and arranged in planar structure over the same substrate(chip), and inductance is determined corresponding to the length of themetallic wiring.

By arranging the separation ends of the loop-shaped primary inductors30-33 and the separation ends of the loop-shaped secondary inductor 35in opposed positions in line, wiring to the inductors 30-33 and to theinductor 35 can be arranged easily.

(An Example of Modification of a Transformer)

FIG. 9 illustrates schematically configuration of a transformer as amodified example, employed in the power amplification circuit accordingto Embodiment 3 of the present invention. The present configuration ofthe transformer 10 illustrated in FIG. 9 exemplifies also the case wherefour differential push-pull amplifiers are employed.

In FIG. 9, loop-shaped wirings 40-43 and 44, each having a partseparated (separation ends), are arranged in lamination. The wirings40-43 are utilized as primary inductors, and the separation ends arecoupled to outputs of the corresponding amplifiers via signal wirings 46a, 46 b-49 a, 49 b, respectively. On the other hand, the wiring 44 isutilized as a secondary inductor and the separated ends are coupled tothe output terminals 4 and 5 via signal wirings 45 a and 45 b,respectively.

In the case of the configuration illustrated in FIG. 9, the wirings40-43 utilized as the primary inductors and the wiring 44 utilized asthe secondary inductor are laminated; accordingly, it is possible toreduce further a layout area of the transformer 10.

In FIG. 9, the wirings 40-43 have the identical shape, and theirmatching frequencies are adjusted with a capacitance value coupled inparallel with the respective wirings. The wirings 40-43 may havedifferent shape, respectively.

In the configuration illustrated in FIGS. 8 and 9, it is necessary totake into consideration magnetic coupling among the primary inductors.Magnetic coupling among the primary inductors can be adjusted by theshape and the spacing of the wirings 40-43 utilized as the primaryinductors, a position of the wiring 44 utilized as the secondaryinductor, and a capacitance value of an individual capacitor coupled toeach wiring. Here, a capacitor may be coupled also in the wiring 44utilized as the secondary inductor.

As described above, in Embodiment 3 of the present invention, thesecondary inductor of the output transformer of the plural differentialpush-pull amplifiers is provided in common to the plural differentialpush-pull amplifiers. Therefore, the arrangement surface area of thetransformer for output matching and output combining can be reduced, andcorrespondingly, the layout area of the power amplification circuit canbe reduced.

Embodiment 4

FIG. 10 illustrates schematically configuration of a power amplificationcircuit according to Embodiment 4 of the present invention. Twodifferential push-pull amplifiers are used in the configuration of thepower amplification circuit illustrated in FIG. 10.

In FIG. 10, the power amplification circuit includes N-channel MOStransistors (insulated-gate field effect transistors) TR11 and TR21 ofwhich the gates are coupled to an input terminal 1, and N-channel MOStransistors TR12 and TR22 of which the gates are coupled to an inputterminal 2. The sources of the MOS transistors TR11 and TR21 aregrounded in common, and the sources of the MOS transistors TR12 and TR22are grounded in common. The MOS transistors TR11, TR12, TR21, and TR22operate as an amplification element respectively, and correspond to theamplifier AMP explained in the above described embodiments. A gate biasvoltage Vg is supplied via a bias resistance Rb to the gates of the MOStransistors TR11, TR12, TR21, and TR22.

The present power amplification circuit further includes at least acapacitor C1 coupled between the drain nodes of the MOS transistors TR11and TR12, a capacitor C2 coupled between the drain nodes of the MOStransistors TR21 and TR22, and a transformer 50 provided with a functionof output combining and output matching.

The transformer 50 includes a primary inductor wiring 52 formed in theshape of a loop with one end separated, a primary inductor wiring 54arranged inside the primary inductor wiring 52 and formed in the shapeof a loop with one end separated, and a secondary inductor wiring 56arranged between the primary inductor wirings 52 and 54, and formed inthe shape of a loop with one end separated at a part in an opposedposition to the separation ends of the primary inductors 52 and 54. Theinductor wirings 52, 54, and 56 are arranged concentrically.

The separation ends of the primary inductor wiring 52 are coupled to thedrain nodes of the MOS transistors TR11 and TR12, respectively, and theseparation ends of the primary inductor wiring 54 are coupled to thedrain nodes of the MOS transistors TR21 and TR22, respectively. Theprimary inductor wirings 52 and 54 are inter-coupled by a center tapwiring 60 at a part in an opposed position to the separation ends (acenter point of the loop-shaped wiring), and drain bias voltage Vd issupplied via the wiring 60.

The secondary inductor wiring 56 has a wider line width than the primaryinductor wirings 52 and 54. The separation ends of the secondaryinductor wiring 56 are coupled to the output terminals 4 and 5,respectively.

Since the source-grounded MOS transistor is generally an invertingamplifier, an output signal OUT(−) is outputted to the output terminal4, and an output signal OUT(+) is outputted to the output terminal 5.The separation ends of the primary inductor and the separation ends ofthe loop-shaped secondary inductor are arranged in the oppositedirection. Therefore, when the polarity of a coil is defined by astarting end in the clockwise direction, the position relationshipbetween the output terminal 4 and the output terminal 5 will becomeopposite to the arrangement illustrated in FIG. 1.

In the configuration of the power amplification circuit illustrated inFIG. 10, one differential push-pull amplifier is comprised of the MOStransistors TR11 and TR12, the capacitor C1, the primary inductor wiring52, and the secondary inductor wiring 56, and another differentialpush-pull amplifier is comprised of the MOS transistor TR21 and TR22,the capacitor C2, the primary inductor wiring 54, and the secondaryinductor wiring 56. Matching frequencies of the two differentialpush-pull amplifiers differ with each other. Since the length of theprimary inductor wiring 54 is shorter than the primary inductor wiring52, when the capacitance of the capacitors C1 and C2 is equal, theresonance frequency of a resonance circuit which is formed by theprimary inductor wiring 54 and the capacitor C2 is higher than theresonance frequency of a resonance circuit which is formed by theprimary inductor wiring 52 and the capacitor C1.

The line width of the secondary inductor wiring 56 is set several times(at least three times) wider than the line width of the primary inductorwirings 52 and 54, and fully suppresses magnetic coupling between theprimary inductor wirings 52 and 54. In this case, it is effective that aspacing between the primary inductor wirings 52 and 54 is set as threeor more times of the line width of the primary inductor wirings 52 and54, from a viewpoint of not increasing the layout area of thetransformer 50 so much but fully suppressing the magnetic couplingbetween the primary inductor wirings 52 and 54.

For example, in the case of a linear inductor wiring (slab inductor),when the wiring space is increased three times of the line width, acoupling coefficient will be reduced roughly by half compared with acase where the wiring space is very narrow.

In the configuration of the power amplification circuit illustrated inFIG. 10, the wiring 60 supplies the drain bias voltage Vd and functionsas virtual AC grounding of each of the two differential push-pullamplifiers. Therefore, even if the primary inductor wirings 52 and 54are inter-coupled by the wiring 60, no adverse influence is exerted on asignal generated by the primary inductor wirings 52 and 54, since thewiring 60 functions as the virtual AC grounding.

In the case of the power amplification circuit illustrated in FIG. 10where two differential push-pull amplifiers are employed, although aneffect of realizing a wider bandwidth of the frequency characteristic isreduced to some extent, chip area reduction effect can be enjoyed,magnetic coupling between the primary inductors can be reduced easily,and the design can be simplified.

In a differential push-pull amplifier, the asymmetry of the circuitarrangement may produce the asymmetry of parasitic components, and maybecome the cause of bringing about output reduction and/or distortionincrease of the amplifier. However, in the configuration of the poweramplification circuit illustrated in FIG. 10, it is possible to arrangethe circuit to a mirror-image symmetry, with respect to a straight linewhich passes along the separation ends of the primary inductor wirings52 and 54 and the separation ends of the secondary inductor wiring 56 ofthe transformer 50. Accordingly, it is possible to eliminate the problemsuch as output reduction and/or distortion increase.

The transformer 50 is also in mirror-image symmetry and the medianpoints of the primary inductor wirings 52 and 54 act as the virtual ACgrounding as described above. The median points of the primary inductorwirings 52 and 54 can be coupled by the center tap wiring 60, and thedrain bias voltage Vd can be supplied from one end of the center tapwiring. Since the center tap wiring 60 acts as the virtual AC grounding,it is not necessary to provide an AC blocking coil for separating theprimary inductor wirings 52 and 54 from a power source (Vd);accordingly, it is possible to simplify the structure of thetransformer, and to reduce the layout area.

(Modification)

FIG. 11 illustrates schematically configuration of a transformer as amodified example, employed in the power amplification circuit accordingto Embodiment 4 of the present invention. In the configurationillustrated in FIG. 11, three differential push-pull amplifiers areprovided in the power amplification circuit. In a transformer 70,loop-shaped primary inductor wirings 72, 74 and 76, each having a partseparated (separation ends) and aligned in line, are arrangedconcentrically. A loop-shaped secondary inductor wiring 60 with a partseparated (separation ends) is arranged between the primary inductorwirings 72 and 74. A loop-shaped secondary inductor wiring 82, with apart separated (separation ends) and aligned with the separation ends ofthe secondary inductor wiring 80, is arranged between the primaryinductor wirings 74 and 76.

The separation ends of the primary inductor wiring 72 are coupled tooutputs of the corresponding amplifier via signal wirings 77 a and 77 b,respectively. The separation ends of the primary inductor wiring 74 arecoupled to outputs of the corresponding amplifier via signal wirings 78a and 78 b, respectively. The separation ends of the primary inductorwiring 76 are coupled to outputs of the corresponding amplifier viasignal wirings 79 a and 797 b, respectively.

The secondary inductor wirings 80 and 82 are short-circuited by signalwirings 84 a and 84 b, at short-circuiting parts 85 a and 85 b which arearranged in an opposed position to the separation ends of the primaryinductor wirings 72, 74, and 76. The signal wirings 84 a and 84 b arecoupled to the output terminals 4 and 5, respectively.

As illustrated in FIG. 11, magnetic coupling between the primaryinductors can be made small by arranging each secondary inductor wiringbetween the primary inductor wirings. By short-circuiting the secondaryinductor wirings 80 and 82 by the signal wirings 84 a and 84 b at theshort-circuiting parts 85 a and 85 b, signals generated in the secondaryinductor wirings 80 and 82 are combined together and transferred to theoutput terminals 4 and 5.

In the arrangement illustrated in FIG. 11, it is possible to realizeconfiguration in which n differential push-pull amplifiers (n=3) areemployed, by arranging the primary inductor wiring and the secondaryinductor wiring alternately.

In the arrangement of the transformer 70 illustrated in FIG. 11, bysetting the line width of the secondary inductor wirings 80 and 82 asseveral times (preferably three or more times) wider than the line widthof the primary inductor wirings 72, 74, and 76, magnetic couplingbetween the primary inductors can be further reduced, similarly as inthe case where two differential push-pull amplifiers are employed, asillustrated in FIG. 10. However, the occupied area of the transformerincreases in the present case.

Also in the configuration of the transformer 70 illustrated in FIG. 11,the drain bias voltage may be supplied at median points of the primaryinductor wirings 72, 74, and 76, near the short-circuiting part 85 a and85 b.

When two differential push-pull amplifiers are employed in thearrangement illustrated in FIG. 11, the transformer 70 may be formedusing the primary inductor wirings 72 and 76, deleting the primaryinductor wiring 74. Also in the present case, it is possible to maintaina sufficient broad spacing between the primary inductor wirings 72 and76, with the help of the secondary inductor wirings 80 and 82, and it ispossible to fully suppress magnetic coupling between the primaryinductor wirings 72 and 76.

As described above, according to Embodiment 4 of the present invention,the loop-shaped secondary inductor wirings are arranged between theloop-shaped primary inductor wirings, and magnetic coupling between theprimary inductor wirings can be reduced. Accordingly, the necessity oftaking into consideration the magnetic coupling between the primaryinductor wirings is reduced, and the design is simplified.

Embodiment 5

FIG. 12 illustrates schematically configuration of a power amplificationcircuit according to Embodiment 5 of the present invention. Although thepower amplification circuit illustrated in FIG. 12 is different from thepower amplification circuit illustrated in FIG. 10 in configuration ofthe transformer 90, but is the same in configuration of the amplifier.Therefore, the same reference number is attached to componentspertaining to the amplifier, and the detailed explanation thereof isomitted.

In FIG. 12, the transformer 90 includes primary inductor wirings 92 and94 formed in the shape of a loop with a part separated (separationends), and secondary inductor wirings 96 and 98 arranged between theprimary inductor wirings 92 and 94. The secondary inductor wirings 96and 98 are coupled in series at a coupling area 100 arrangedcorresponding to the separation ends of the primary inductor wirings 92and 94. The primary inductor wirings 92 and 94 are inter-coupled at theseparation ends of the secondary inductor wiring 96 by a center tapwiring 102, and are supplied with drain bias voltage Vd.

In the configuration of the transformer 90 illustrated in FIG. 12, theprimary inductor wirings 92 and 94 and the secondary inductor wirings 96and 98 have the same line width. A spacing between the primary inductorwirings 92 and 94 is at least three times wider than the line width, andthe magnetic coupling is rendered small. Since the secondary inductorwirings 96 and 98 are coupled in series, load resistance of eachdifferential amplifier is divided and chip area reduction effect isrealized, furthermore, it is possible to reduce drain voltage of thetransistors TR11, TR12, TR21, and TR22 of the amplifier. (This isbecause a voltage twice as high as the applied voltage of the primarytransformer wiring can be generated between the output terminals 4 and5).

Also in a case where the spacing between the secondary inductor wirings96 and 98 is narrowed, owing to the impedance conversion by thetransformer 90 having the turn ratio 1:2 of the primary inductor and thesecondary inductor, it is possible to obtain the output voltage fourtimes as high as the input voltage, and as a result, it is possible toreduce drain voltage of the MOS transistors TR11, TR12, TR21, and TR22of the amplifier.

The cross structure at the coupling area 100 of the secondary inductorwirings 96 and 98 may utilize the same configuration of the structure inthe cross point of the secondary inductor wiring 98 and the center tapwiring 102. Namely, for example, in the coupling area 100, theconfiguration in which the secondary inductor wirings 98 and 96 arelaminated through the intermediary of an insulator layer may beutilized.

As described above, according to Embodiment 5 of the present invention,the secondary inductor wiring is arranged between the primary inductorwirings, accordingly, it is possible to reduce magnetic coupling betweenthe primary inductors, and it is possible to provide a transformerhaving of a small amount of signal interference and a small occupiedarea.

In addition, by arranging and coupling in series the plural secondaryinductor wirings, the drain voltage of a transistor of the amplifier canbe reduced by a turn ratio of the primary inductor and the secondaryinductor.

When applied to a power amplification circuit used in a field where aflat frequency characteristic is required over wide bands, such as amobile communications field, the present invention can realize a poweramplification circuit which has a wide band frequency characteristic bya simple circuit configuration. The differential push-pull amplifier maybe integrated over a common substrate, or alternatively, the transformermay be formed over a common substrate, and components other than thetransformer of the differential push-pull amplifier may be formed overanother substrate.

1. A power amplification circuit comprising: a plurality of differentialpush-pull amplifiers coupled to a common input terminal, each coupled toan output terminal in common through a transformer for output matching,the plurality of differential push-pull amplifiers being matched atmutually different frequencies, wherein output signals of the pluralityof differential push-pull amplifiers are combined together by asecondary inductor of the transformer.
 2. The power amplificationcircuit according to claim 1, wherein each of the differential push-pullamplifiers includes: a primary inductor provided as a part of thetransformer; a preceding-stage amplifier operable to receive an inputsignal; a succeeding-stage amplifier operable to receive and perform aninverting amplification of an output signal of the preceding-stageamplifier to drive the primary inductor; and a feedback resistor coupledbetween an input and an output of the succeeding-stage amplifier.
 3. Thepower amplification circuit according to claim 1, wherein the inputterminal is supplied with a differential signal, and wherein each of thedifferential push-pull amplifiers includes: a first preceding-stageamplifier operable to receive a first signal of the differential signal;a second preceding-stage amplifier operable to receive a second signalcomplementary to the first signal of the differential signal; a firstsucceeding-stage amplifier operable to perform non-invertingamplification of an output signal of the first preceding-stageamplifier; a second succeeding-stage amplifier operable to performnon-inverting amplification of an output signal of the secondpreceding-stage amplifier; a first feedback resistor coupled between aninput of the first succeeding-stage amplifier and an output of thesecond preceding-stage amplifier; a second feedback resistor coupledbetween an input of the second succeeding-stage amplifier and an outputof the first succeeding-stage amplifier; and a primary inductor coupledbetween the output of the first succeeding-stage amplifier and theoutput of the second succeeding-stage amplifier.
 4. The poweramplification circuit according to claim 1, wherein the transformer foroutput matching includes: a plurality of primary inductors each providedcorrespondingly to each of the differential push-pull amplifiers; and asecondary inductor provided in common to the differential push-pullamplifiers and magnetically coupled with the primary inductors.
 5. Thepower amplification circuit according to claim 4, wherein the pluralityof primary inductors have loop-shaped primary inductor wirings eacharranged concentrically in order, and wherein the secondary inductor hasa secondary inductor wiring arranged in the shape of a loop surroundingthe primary inductor wirings.
 6. The power amplification circuitaccording to claim 4, wherein the primary inductors and the secondaryinductor have loop-shaped inductor wirings arranged in lamination. 7.The power amplification circuit according to claim 1, wherein theplurality of differential push-pull amplifiers includes first and seconddifferential push-pull amplifiers each having a primary inductor of thetransformer and being matched at mutually different frequencies, andwherein, in the transformer for output matching, the primary inductorprovided for the first differential push-pull amplifier is comprised ofa first loop-shaped primary inductor wiring formed in the shape of aloop, the primary inductor provided for the second differentialpush-pull amplifier is comprised of a second loop-shaped primaryinductor wiring arranged on the inner side of the first loop-shapedprimary inductor wiring, the secondary inductor is comprised of aloop-shaped secondary inductor wiring arranged between the firstloop-shaped primary inductor wiring and the second loop-shaped primaryinductor wiring, and the spacing between the first loop-shaped primaryinductor wiring and the second loop-shaped primary inductor wiring is atleast three times as much as a line width of each of the first andsecond primary inductor wirings.
 8. The power amplification circuitaccording to claim 7, wherein the first and second differentialpush-pull amplifiers include a differential amplifier stage operable toamplify a differential signal supplied to the input terminal, whereineach of the first and second loop-shaped primary inductor wiringsincludes a respective separation end coupled to an output of thecorresponding differential amplifier stage, and wherein respective partsof the first and second loop-shaped primary inductor wirings and thesecond loop-shaped primary inductor wiring in an opposed position to therespective separation ends are short-circuited by a center tap wiring.9. The power amplification circuit according to claim 7, wherein theloop-shaped secondary inductor wiring includes first and secondloop-shaped secondary inductor wirings each arranged between the firstand second loop-shaped primary inductor wirings and each having aseparation part, and the first and second loop-shaped secondary inductorwirings are coupled in parallel at ends of the respective separationparts.
 10. The power amplification circuit according to claim 7, whereinthe number of turns of the secondary inductor is greater than the numberof turns of each of the first primary inductor and the second primaryinductor.